Powdery Mildew is Not a Systemic Pathogen

Photo by Gabe Gruye, Knights Hemplar

Forums provide valuable information for growers, but in some cases the information is misleading. I visit smart, diligent cannabis growers. Regardless of growing environment or strains, you all battle powdery mildew during the crop cycle. Growers often ask if powdery mildew is a systemic fungal pathogen.

It is not.

Much like cannabis, grapes also harbor powdery mildew. Powdery Mildew infested grapes can spoil wine because the spores stop fermentation. Viticulturists have a low tolerance for infestation. When I make powdery mildew management recommendations, I look to the active ingredients applied in vineyards. With one exception, you cannot apply systemic fungicides. No Eagle 20 (myclobutanil) should be applied to cannabis at any time, period.

Here’s why Eagle 20 is a bad idea.

Downy mildew infested rose

Systemic fungal pathogens live within the host and are able to grow away from the site of infection to other parts such as leaves, and stems, and into roots, rhizomes and stolons. (1) Downy Mildew (Peronospora sp) is a water borne plant disease that can infest the vascular cambium (central vascular stem or pith) of plants. This pathogen can be nearly impossible to control. Unlike most fungal pathogens, powdery mildew fungi tend to grow superficially, or epiphytically, on plant surfaces. (2) The classic white powdery infestation occurs on the plant surface, and the mycelium invades the leaf one cell deep (Jessica Cortell PhD). Non systemic fungal pathogens live in or on a host, and form localised lesions. They can’t grow away from the site of infection. (3)

Photo by Gabe Gruye Knights Hemplar

Growers can manage powdery mildew by understanding the disease is a contaminant in the growing environment. You can eradicate (kill) spores using Zerotol (Hydrogen Dioxide & Peroxyacetic Acid) and Milstop (Potassium bicarbonate) followed by applications of biological fungicides such as Bacillus subtilis. Apply state approved EPA registered products in your grow facility. Use plant protection products safely. Always read the label and product information before use. Pay attention to the risk indications and follow the safety precautions on the label.